One of the prerequisites to extended shelf life of electrical energy storage devices, such as batteries or condensers, is the integrity of the package or container enclosing the electrochemically active materials in the device. A leak, however minute or undetectable at the time of manufacture, will permit water from the electrolyte to evaporate at an undiscernible rate, resulting in a slow but continuous diminution in the electrical properties of the battery, resulting eventually in a low capacity or even in total failure. In addition, the entry of oxygen will deleteriously affect the condenser's or the battery's properties.
While such a problem of leakage is not as common in devices enclosed in a "can", such as the well known C and D cells, such leakage can occur. However, the leakage or pinhole problem is more widespread in the flat or planar batteries which are now being widely used. Such batteries are disclosed in, for example, U.S. Pat. Nos. 3,563,805; 3,617,387; 3,734,780; 3,770,504; and the like. Such planar batteries are generally enclosed in an envelope that has been suitably rendered air and moisture resistant by lamination with suitable polymeric materials, metal foils, wax-dipping and the application of other impermeable coatings. Similar problems are encountered in flat condensers.
That the problem of leakage is a serious one is attested to by the number of references in the literature to various adhesives and sealants which have been employed to alleviate this problem. However, as stated above, even though careful precautions have been taken and manufacturing procecures followed, batteries are still produced which upon manufacture appear perfectly acceptable, but which upon storage of several months are found to have failed. If the battery has been incorporated into a photographic film assembly as constituent component, for instance, as disclosed in U.S. Pat. No. 3,543,662, and then found, upon attempted usage of the film product, to have failed, then a greater loss has occurred than merely the loss attributed to a defective battery. It is highly desirable, therefore, to obtain a method for ascertaining as soon as possible after manufacture whether or not the battery will survive an extended shelf life.
A novel method for determining seal quality and hence the shelf life stability of an electrical energy storage device has now been found.